Perovskite solar cell configurations

ABSTRACT

Various perovskite solar cell embodiments include a flexible metal substrate (e.g., including a metal doped TiO2 layer), a perovskite layer, and a transparent electrode layer (e.g., including a dielectric/metal/dielectric structure), wherein the perovskite layer is provided between the flexible metal substrate and the transparent electrode layer. Also, various tandem solar cell embodiments including a perovskite solar cell and either a quantum dot solar cell, and organic solar cell or a thin film solar cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) from U.S.provisional patent application No. 62/536,111, entitled “PerovskiteSolar Cell Configurations” and filed on Jul. 24, 2017, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to solar cells, and, in particular, to anumber of perovskite solar cell configurations, such as, withoutlimitation, flexible and stable perovskite solar cells.

2. Description of the Related Art

Solar energy has attracted huge attention as a promising alternative tofossil fuel energy (which is regarded as a major source of thegreen-house effect). However, to replace fossil fuel energy on aworthwhile scale, the manufacturing costs of solar cells need to bedecreased and/or the power conversion efficiency (PCE) of solar cellsneeds to be increased. Recently, organic-inorganic perovskitesemiconductors, such as halide perovskite (CH₃NH₃PbX₃, X═halogen ions),have been found to be an excellent light absorber for use in creatinghighly efficient and economically viable solar cells, known as halideperovskite solar cells (PSCs). Halide perovskites have outstandingoptical and electronic properties, such as a high absorption coefficient(>10⁴ cm⁻¹), a long carrier diffusion length (>1 μm), high carriermobility (25 cm²/Vs), and a suitable band gap spanning the energy ofvisible and near infrared light. In addition, simple and cheap synthesisprocesses can be applied to the large scale production of halide PSCs.Thus, this emerging hybrid solar cell has the potential to meet theurgent need for low cost and high efficiency power generation.

In a typical, known PSC device, a perovskite layer is coated on top of amesoporous or planar titanium oxide (TiO₂) layer that is provided on arigid transparent conducting oxide (TCO) substrate, such as indium tinoxide (ITO)/glass or fluorine doped tin oxide (FTO)/glass. Recentstudies have shown that highly crystalline TiO₂ is necessary as anelectron transport layer (ETL) for high efficiency PSCs. It is knownthat highly crystalline TiO₂ may be obtained by annealing the TiO₂ layerat a high temperature (e.g., 500° C.) before the perovskite layer iscoated onto the TiO₂ layer. This process, however, cannot be applied toflexible polymer substrates, such as polyethylene terephthalate (PET)and polyethylene naphthalate (PEN), that are suitable for flexibleelectronics. To address this problem, the fabrication of n-typematerials at low temperature (under 150° C.) has been explored.Specifically, in one example application, a low-temperature processedTiOx compact layer was deposited on an indium tin oxide/polyethylenenaphthalate (ITO/PEN) substrate through atomic layer deposition (ALD) at80° C., achieving a high power conversion efficiency (PCE) of 12.2%. :Ithas also been reported that amorphous TiO₂ grown by magnetron sputteringat room temperature increases the PCE of PSCs on ITO/PEN to 15.07%.Moreover, in recent years, flexible PSCs of an inverted structure(p-i-n) were designed. The flexible inverted PSCs are composed of NiOx,PhNa-1T, or PEDOT: PSS as a p-type hole transport layer (HTL) onITO/polymer substrates. The PCE of flexible inverted PSCs was only ableto reach about 14.7%, which is not as good as the best performing PSCson rigid substrates (>22%). Recent results indicate that there is alimitation in increasing the PCE of flexible PSCs without usinghigh-temperature annealing of the TiO₂ layer. In addition, themechanical strength of PSCs on ITO/polymer substrates is weak due to lowfatigue resistance of thick ITO layers (ca. 200 nm). Since Poisson'sratio of ITO is smaller than that of CH3NH3PbI3, ITO is less bendablethan CH3NH3PbI3. Consequently, as PSCs are bent repeatedly, a crack isformed in the ITO layer that propagates through the perovskite layer,resulting in the degradation of the PSCs.

There are several alternatives to ITO/plastic substrates, such asgraphene coated polymers, surface treated metal plates and metal meshes.Among these alternative materials, a metal plate is an attractivecandidate, due to the capability of high temperature annealing, lowmanufacturing cost, and excellent mechanical properties. In fact, thereare several studies on PSCs and dye sensitized solar cells having atitanium metal plate as a substrate, but they are coated with anadditional ETL instead of using the oxidized surface layer of Ti, andthe PCE of these PSCs is less than 11%. Recently, a metal Ti film onFTO/glass substrate was oxidized to form an ETL of PSCs. However, suchPSCs are not flexible and the PCE of such PSCs built on a surfaceoxidized Ti film/PTO/glass substrate has been only about 13%. Inaddition, this type of PSC suffers from hysteresis, which is attributedto the poor quality of ETL.

SUMMARY OF THE INVENTION

In one embodiment, a perovskite solar cell is provided that includes aflexible metal substrate including a metal foil layer, a perovskitelayer, and a transparent electrode layer including adielectric/metal/dielectric structure. The perovskite layer is providedbetween the flexible metal substrate and the transparent electrodelayer, and the transparent electrode layer enables illumination of theperovskite layer through the transparent electrode layer.

In another embodiment, a perovskite solar cell is provided that includesa flexible metal substrate including a metal doped TiO₂ layer, aperovskite layer, and a transparent electrode layer, wherein theperovskite layer is provided between the flexible metal substrate andthe transparent electrode layer.

In still another embodiment, a method of making a perovskite solar cellis provided. The method includes providing a foil layer, providing alayer of metal on top of the Ti foil layer, thermally oxidizing the Tifoil layer and the layer of metal to form a metal doped TiO₂ layer,providing a perovskite layer on top of the metal doped TiO₂ layer.

In yet another embodiment, a tandem solar cell is provided that includesa perovskite solar cell as described above, and a quantum dot solar cellor an organic solar cell. If a quantum dot solar cell is used, itincludes a quantum dot layer, where the quantum dot layer is provided ontop of the transparent electrode layer of the perovskite solar cell. Ifan organic solar cell is used, it includes a polymer layer, where thepolymer layer is provided on top of the transparent electrode layer ofthe perovskite solar cell. A second transparent electrode layerincluding a dielectric/metal/dielectric structure is also included,wherein the second transparent electrode layer is provided on top of thequantum dot layer or the polymer layer.

In a further embodiment, an alternative tandem solar cell is providedthat includes a thin-film solar cell, a first transparent electrodelayer including a dielectric/metal/dielectric structure, wherein thefirst transparent electrode layer is provided on top of the thin-filmsolar cell, a perovskite layer provided on top of the first transparentelectrode layer, and a second transparent electrode layer including adielectric/metal/dielectric structure, wherein the second transparentelectrode layer is provided on top of the thin-film solar cell.

In another embodiment, an alternative perovskite solar cell is providedthat includes a substrate, a perovskite layer provided on the substrate,and a hole transport material layer provided on the perovskite layer,wherein the perovskite layer and the hole transport material layertogether have a matched work function (i.e., a first work function) forhole transport from the perovskite layer to the hole transport layer.The perovskite solar cell further includes a transparent electrode layerincluding a dielectric/metal/dielectric structure, wherein theperovskite layer and the hole transport material layer are providedbetween the substrate and the transparent electrode layer, wherein thework function of the dielectric/metal/dielectric structure (i.e., asecond work function) facilitates the hole transport from the holetransport layer to the transparent layer of thedielectric/metal/dielectric structure and matches the first workfunction.

In still a further embodiment, a method of making a perovskite solarcell including a perovskite layer, a hole transport material layer and atransparent electrode layer including a dielectric/metal/dielectricstructure is provided, wherein the perovskite layer and the holetransport material layer together have a first work function. The methodincludes selecting dielectric and metal materials for thedielectric/metal/dielectric structure so that thedielectric/metal/dielectric structure has a second work function thatmatches the first work function, providing a substrate, providing theperovskite layer and the hole transport material layer on the substrate,and providing the transparent electrode layer on the perovskite layerand the hole transport material layer such that the perovskite layer andthe hole transport material layer are provided between the substrate andthe transparent electrode layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a flexible perovskite solar cell according toan exemplary embodiment of the disclosed concept;

FIG. 2 is a schematic diagram of a tandem solar cell according to analternative embodiment of the disclosed concept;

FIG. 3 is a schematic diagram of a tandem solar cell according to afurther alternative embodiment of the disclosed concept; and

FIG. 4 is a schematic diagram of a tandem solar cell according to stilla further alternative embodiment of the disclosed concept.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As used herein, the singular form of “a”, “an”, and “the” include pluralreferences unless the context clearly dictates otherwise.

As used herein, the statement that two or more parts or components are“coupled” shall mean that the parts are joined or operate togethereither directly or indirectly, i.e., through one or more intermediateparts or components, so long as a link occurs.

As used herein, “directly coupled” means that two elements are directlyin contact with each other.

As used herein, the term “number” shall mean one or an integer greaterthan one (i.e., a plurality).

As used herein, the term “perovskite” shall mean an organic-inorganichalide compound with a perovskite crystal structure.

As used herein, the term “dielectric/metal/dielectric structure” shallmean a structure where a metal is provided between two dielectricmaterials (e.g., a top dielectric material and a bottom dielectric) in amultilayer structure. In one particular, non-limiting implementation ofa dielectric/metal/dielectric structure, the metal is provided directlyon the top surface of the bottom dielectric material and the topdielectric material is provided directly on the top surface of themetal.

As used herein_(;) the term “nanoscale” shall mean an object having asize (e.g., diameter or width) ranging from 1 nm to 1 μm (1,000 nm).

As used herein, the term “quantum dot” shall mean a nanoscalesemiconductor particle.

As used herein, the term “quantum dot layer” shall mean a layer ofmaterial including a number of quantum dots.

As used herein, the term “thin-film solar cell” shall mean a solar cellhaving one or more thin layers/thin-films of photovoltaic materialprovided on a substrate, such as silicon (Si), glass, plastic or metal.The photovoltaic material may include, for example and withoutlimitation, amorphous thin-film Si, cadmium telluride (CdTe), or copperindium gallium selenide (CIGS).

As used herein, the term “organic solar cell” shall mean a solar cellhaving one or more layers of photovoltaic material in the form of aconductive organic polymer for light absorption and charge transport toproduce electricity provided on a substrate, such as a polymersubstrate.

As used herein, the term “provided on” or “provided on top of” shallmean that an element or material is (i) provided directly on or on topof another element or material, or (ii) provided indirectly on or on topof another element or material with one or more intervening elements ormaterials being provided between the element or material and the anotherelement or material.

Directional phrases used herein, such as, for example and withoutlimitation, top, bottom, left, right, upper, lower, front, back, andderivatives thereof, relate to the orientation of the elements shown inthe drawings and are not limiting upon the claims unless expresslyrecited therein.

The disclosed concept will now be described, for purposes ofexplanation, in connection with numerous specific details in order toprovide a. thorough understanding of the subject innovation. It will beevident, however, that the disclosed concept can be practiced withoutthese specific details without departing from the spirit and scope ofthis innovation.

FIG. 1 is a schematic diagram of a flexible perovskite solar cell 2according to an exemplary embodiment of the disclosed concept. As seenin the FIG. 1, perovskite solar cell 2 is a multi-layer structure, eachlayer of which is described in detail below.

Referring to FIG. 1, perovskite solar cell 2 includes a flexible metalsubstrate assembly 4 that comprises two parts: (i) a titanium foil layer6, and (ii) a metal doped TiO₂ layer 8 provided on the top surface oftitanium foil layer 6. In the exemplary embodiment, metal substrateassembly 4 is formed as follows. First, a thin (e.g., 0.5-5 nm) layer ofa metal, such as, without limitation, aluminum (Al), tantalum (Ta), iron(Fe) or niobium (Nb), is coated onto the top surface of a titanium foilusing a suitable method, such as e-beam evaporation, thermal evaporationor sputtering. The coated titanium foil is then thermally oxidized(annealed) at a high temperature (e.g., 300-700° C.) in a firstannealing step. Ih the exemplary embodiment, the coated titanium foil isthen further annealed at high temperature (e.g., 400° C.) in ambientoxygen. These annealing steps cause the coated metal to be doped intothe oxide being formed from the titanium foil, which results in theformation of metal doped TiO₂ layer 8 on top of titanium foil layer 6 asshown in FIG. 1. The metal doped TiO₂ layer 8 as just described isbeneficial because it increases current density of solar cells andeliminates hysteresis effects that are otherwise present in known PSCdevices. More specifically, as described elsewhere herein, in aconventional PSC structure, TiO₂ nanoparticles are coated onto thesurface of a transparent electrode coated glass as an electron transportlayer. Such a TiO₂ nanoparticle layer, however, causes hysteresis of theJ-V curve of the PSC device due to a dependence of electric currentdensity on the electric field speed and direction. The metal doped TiO₂layer 8 of the disclosed concept can eliminate this hysteresis effect byreducing the detect density of TiO₂ near the perovskite/TiO₂ interfaceand changing the surface charge status of the TiO₂ layer.

Perovskite solar cell 2 further includes a perovskite layer 10 that isprovided on the top surface of metal substrate assembly 4. In thenon-limiting exemplary embodiment, perovskite layer 10 is a layer ofCH₃NH₃PbI₃ that is coated (using a suitable method such as spin coating)onto the top surface of metal substrate assembly 4, although otherperovskites are contemplated within the scope of the disclosed concept.A hole transport material (HTM) layer 12 is provided on the top surfaceof perovskite layer 10 using a process such as spin coating. In thenon-limiting exemplary embodiment, hole transport material layer 12 ismade of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS), although other suitable materials are contemplated withinthe scope of the disclosed concept.

Finally, a transparent electrode layer 14 is provided on the top surfaceof hole transport material layer 12. In the exemplary environment,transparent electrode layer 14 is a flexible dielectrichnetal/dielectiicmultilayer structure wherein a metal material is sandwiched between twodielectric materials. In the exemplary embodiment, the three layers ofthe flexible dielectric/metal/dielectric multilayer structure aresequentially deposited using a physical vapor deposition method such ase-beam evaporation. The dielectric/metal/dielectric multilayer structuremay be a number of different material combinations such as, withoutlimitation, MoO/Au/MoO, TiO₂/Au/MoO, ZnO/Ag/NiO, MoO/Al/MoO orMoO/Ag/NiO.

Because both metal substrate assembly 4 and transparent electrode layer14 in the form of the dielectric/metal/dielectric multilayer structureare flexible, perovskite solar cell 2 as a whole is veryflexible/bendable and may be used in applications, such as wearableelectronics, which require flexible components. This is in contrast toconventional PSC structures which, as described elsewhere herein, usetransparent electrodes and/or coated glass substrates that are rigid. Infact, the present inventors have determined through experimental testingthat perovskite solar cell 2 as described herein will not show cracks ordegraded PCE after repeated bending (e.g., one test that was performedwas a 1000 time-cyclic bending test at a bending radius of 4 mm).

Furthermore, it has been determined that perovskite solar cell 2demonstrates enhanced stability under light illumination. In particular,aging of conventional PSCs under UV illumination is well-known and isattributed to the photovoltaic effect of UV light on the TiO₂ in suchconventional PSCs. Perovskite solar cell 2 of the disclosed conceptresolves this photo-aging problem by changing the light illuminationdirection to a top-illumination direction. In particular, in contrast toa conventional PSC structure wherein the light illumination is from thebottom direction, in perovskite solar cell 2, light is incident from thetop direction through transparent electrode layer 14 comprising thedielectric/metal/dielectric multilayer structure. Since perovskite layer10 in perovskite solar cell 2 will absorb UV light, metal doped TiO₂layer will not be exposed to U V light and, as a result, reactiveradical ions will not be produced during the solar power/electricityconversion process.

A further advantage of perovskite solar cell 2 is the fact thattransparent electrode layer 14 that includes thedielectric/metal/dielectric multilayer structure functions as apassivation layer that prevents undesired aging of perovskite solar cell2 due to humidity and oxygen in the air. Aging of current prior art PSCsdue to humidity and oxygen in the air is a substantial problem whichprevents commercialization of PSCs. According to an aspect of thedisclosed concept, perovskite layer 10 is passivated by transparentelectrode 14 such that perovskite layer 10 does not come into contactwith air and such that water does not penetrate into the perovskitelayer 10. As a result, undesirable aging of perovskite solar cell 2 isprevented. Furthermore, in a case where multiple perovskite solar cells2 are used within a structure, the space between such unit cells may becovered by a dielectric layer using known deposition techniques such ase-beam deposition and atomic layer deposition, thereby increasing thepassivation and protection of each of the unit cells.

One particular exemplary embodiment of the disclosed concept involvescontrolling the work function of transparent electrode layer 14 bydesigning the dielectric/metal/dielectric structure thereof in order tomaximize electron flow and minimize electron accumulation in thesemiconductor side of the semiconductor-electrode junction. As is knownin the art, work function indicates the energy (for example, in Joules)that is required to excite electrons from Fermi level to vacuum level.The work function difference between two materials represents theeasiness of electron (or hole) flow from one material to the other. Inthis particular exemplary embodiment, the specific materials of thedielectric/metal/dielectric structure of transparent electrode layer 14are chosen so that transparent electrode layer 14 has a work functionthat is tuned to “match” the work function of the combination ofperovskite layer 10 and hole transport material layer 12. As usedherein, the work functions of two materials will be considered to“match” one another if the work function of one of the materials iswithin 15% of the work function of the other of the materials.

FIG. 2 is a schematic diagram of a tandem solar cell 16 according to analternative embodiment of the disclosed concept. Tandem solar cell 16 isa stacked solar cell structure that includes a quantum dot solar cell 18stacked on top of a perovskite solar cell 2 as described herein. Morespecifically, as seen in FIG. 2, quantum dot solar cell 18 includes aquantum dot layer 20, which, in the exemplary illustrated embodiment, isa lead sulfide (PbS) quantum dot layer, and a transparent electrode 14as described elsewhere herein that is provided on the top surface ofquantum dot layer 20. Quantum dot solar cell 18 is structured to absorbphotons 0having a first bandgap energy (e.g., a high bandgap energy),and pass photons having a different, second bandgap energy (e.g., a lowbandgap energy). Thus, in operation, as light illuminates tandem solarcell 16, photons having the first bandgap energy will be absorbed byquantum dot solar cell 18, and photons having the second bandgap energythat are not absorbed by quantum solar cell 18 will be absorbed byperovskite solar cell 2. Tandem solar cell 16 thus increases theabsorption efficiency as compared to a solar cell structure includingonly a single type of solar cell.

FIG. 3 is a schematic diagram of a tandem solar cell 22 according to afurther alternative embodiment of the disclosed concept. Tandem solarcell 22 is similar to tandem solar cell 16, except that tandem solarcell 22 includes an organic solar cell 24 instead of a quantum dot solarcell 18.

FIG. 4 is a schematic diagram of a tandem solar cell 26 according tostill a further alternative embodiment of the disclosed concept. Asdescribed in detail below, tandem solar cell 26 is athin-film-perovskite tandem cell structure. Tandem solar cell 26includes a thin-film solar cell 28 which, in the non-limiting exemplaryembodiment, is a Si thin-film solar cell wherein a thin-film ofamorphous Si is deposited on a glass substrate. A transparent electrode14 as described elsewhere herein is provided on the top surface ofthin-film solar cell 28. A perovskite layer 10 as described elsewhereherein is provided on the top surface of transparent electrode 14, and ahole transport material layer 12 as described elsewhere herein isprovided on the top surface of perovskite layer 10. Finally, a secondtransparent electrode 14 as described elsewhere herein is provided onthe top surface of hole transport to layer 12. In tandem solar cell 26,the first transport electrode layer 14 functions as an interconnectionfor the device.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” or “including”does not exclude the presence of elements or steps other than thoselisted in a claim. In a device claim enumerating several means, severalof these means may be embodied by one and the same item of hardware. Theword “a” or “an” preceding an element does not exclude the presence of aplurality of such elements. In any device claim enumerating severalmeans, several of these means may be embodied by one and the same itemof hardware. The mere fact that certain elements are recited in mutuallydifferent dependent claims does not indicate that these elements cannotbe used in combination,

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to he understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

What is claimed is:
 1. A perovskite solar cell, comprising: a flexiblemetal substrate including a metal foil layer; a perovskite layer; and atransparent electrode layer including a dielectric/metal/dielectricstructure, wherein the perovskite layer is provided between the flexiblemetal substrate and the transparent electrode layer, and wherein thetransparent electrode layer enables illumination of the perovskite layerthrough the transparent electrode layer.
 2. The perovskite solar cellaccording to claim 1, wherein the metal foil layer is a titanium foillayer.
 3. The perovskite solar cell according to claim 1, wherein themetal of the dielectric/metal/dielectric structure is Au, Al or Ag. 4.The perovskite solar cell according to claim 1, wherein thedielectric/metal/dielectric structure includes at least one of TiO₂,ZnO, MoO and NiO.
 5. The perovskite solar cell according to claim 1,wherein the dielectric/metal/dielectric structure comprises MoO/Au/MoO,TiO₂/Au/MoO, ZnO/Ag/NiO, MoO/Al/MoO or MoO/Ag/NiO.
 6. A perovskite solarcell, comprising: a flexible metal substrate including a metal dopedTiO₂ layer; a perovskite layer; and a transparent electrode layer,wherein the perovskite layer is provided between the flexible metalsubstrate and the transparent electrode layer.
 7. The perovskite solarcell according to claim 6, wherein the metal doped TiO₂ layer is analuminum doped TiO₂ layer.
 8. The perovskite solar cell according toclaim 6, wherein the metal doped TiO₂ layer is a niobium doped TiO₂layer.
 9. The perovskite solar cell according to claim 6, wherein thetransparent electrode layer includes a dielectric/metal/dielectricstructure.
 10. The perovskite solar cell according to claim 6, whereinthe flexible metal substrate includes a Ti foil layer, and wherein themetal doped TiO₂ layer is provided on the Ti foil layer.
 11. A method ofmaking a perovskite solar cell, comprising: providing a Ti foil layer;providing a layer of metal on top of the Ti foil layer; thermallyoxidizing the Ti foil layer and the layer of metal to form a metal dopedTiO₂ layer; and providing a perovskite layer on top of the metal dopedTiO₂ layer.
 12. The method according to claim 11, wherein the perovskitelayer is provided directly on top of the metal doped TiO₂ layer.
 13. Themethod according to claim 11, further comprising providing a holetransport material layer on top of the perovskite layer and providing atransparent electrode layer including a dielectric/metal/dielectricstructure on top of the hole transport layer.
 14. The method accordingto claim 11, wherein the layer of metal is a layer of aluminum,tantalum, iron or niobium, and wherein the metal doped TiO₂ layer is analuminum doped TiO₂ layer, a tantalum doped TiO₂ layer, an iron dopedTiO₂ layer, or a niobium doped TiO₂ layer.
 15. The method according toclaim 11, wherein the thermally oxidizing comprises annealing the Tifoil layer and the layer of metal for a first period at a firsttemperature, and annealing the Ti foil layer and the layer of metal inambient oxygen for a second period following the first period at asecond temperature.
 16. A tandem solar cell, comprising: a perovskitesolar cell according to claim 1; and a second solar cell comprising oneof: (i) a quantum dot solar cell comprising: a quantum dot layer, wherethe quantum dot layer is provided on top of the transparent electrodelayer of the perovskite solar cell; and a second transparent electrodelayer including a dielectric/metal/dielectric structure, wherein thesecond transparent electrode layer is provided on top of the quantum dotlayer, or (ii) an organic solar cell comprising: a polymer layer, wherethe polymer layer is provided on top of the transparent electrode layerof the perovskite solar cell; and a second transparent electrode layerincluding a dielectric/metal/dielectric structure, wherein the secondtransparent electrode layer is provided on top of the polymer layer. 17.The tandem solar cell according to claim 16, wherein the second solarcell is the quantum dot solar cell.
 18. The tandem solar cell accordingto claim 17, wherein the quantum dot layer comprises a PbS quantum dotlayer.
 19. The tandem solar cell according to claim 16, wherein theflexible metal substrate of the perovskite solar cell includes a metaldoped TiO₂ layer provided on top of the metal foil layer.
 20. A tandemsolar cell, comprising: a thin-film solar cell; a first transparentelectrode layer including a dielectric/metal/dielectric structure,wherein the first transparent electrode layer is provided on top of thethin-film solar cell; a perovskite layer provided on top of the firsttransparent electrode layer; and a second transparent electrode layerincluding a dielectric/metal/dielectric structure, wherein the secondtransparent electrode layer is provided on top of the thin-film solarcell.
 21. The tandem solar cell according to claim 20, wherein the thinfilm solar cell is a Si solar cell.
 22. A perovskite solar cell,comprising: a substrate; a perovskite layer provided on the substrate; ahole transport material layer provided on the perovskite layer, whereinthe perovskite layer and the hole transport material layer together havea first work function; and a transparent electrode layer including adielectric/metal/dielectric structure, wherein the perovskite layer andthe hole transport material layer are provided between the substrate andthe transparent electrode layer, wherein the dielectric/metal/dielectricstructure has a second work function, and wherein the second workfunction matches the first work function.
 23. A method of making aperovskite solar cell including a perovskite layer, a hole transportmaterial layer and a transparent electrode layer including adielectric/metal/dielectfic structure, wherein the perovskite layer andthe hole transport material layer together have a first work function,the method comprising: selecting dielectric and metal materials for thedielectric/metal/dielectric structure so that thedielectric/metal/dielectric structure has a second work function thatmatches the first work function; providing a substrate; providing theperovskite layer and the hole transport material layer on the substrate;and providing the transparent electrode layer on the perovskite layerand the hole transport material layer such that the perovskite layer andthe hole transport material layer are provided between the substrate andthe transparent electrode layer.
 24. A method of making metal dopedTiO₂, comprising: providing a Ti foil layer; providing a layer of metalon top of the Ti foil layer; and thermally oxidizing the Ti foil layerand the layer of metal to form a metal doped TiO₂ layer.
 25. The methodaccording to claim 24, wherein the layer of metal is a layer ofaluminum, tantalum, iron or niobium, and wherein the metal doped TiO₂layer is an aluminum doped TiO₂ layer, a tantalum doped TiO₂ layer, aniron doped TiO₂ layer, or a niobium doped TiO₂ layer.
 26. The methodaccording to claim 24, wherein the thermally oxidizing comprisesannealing the Ti foil layer and the layer of metal for a first period ata first temperature, and annealing the Ti foil layer and the layer ofmetal in ambient oxygen for a second period following the first periodat a second temperature.